It is an ad, but it's also an announcement with information that many hners will be interested in and enjoy discussing. There are links to tools and blog posts on the topic as well as links to the advertised product. I don't think it's out of place here.
Because you have to pay to get the actual content. The title says “How integers and floats work.”But it isn’t a post about how integers and floating point precision numbers are represented in binary. Rather it’s a post announcing an illustrated guide to ints and floats, which you have to pay for.
The submitted title was "How integers and floats work" but really it should have included the "New zine" part. I agree that that changes the meaning and that an article with the submitted title ought to be about how integers and floats work.
> Curious as to why you thought that this was an ad?
Because it is clearly an ad?
It explains what will you get and at what price. It does not explain "how Integers and Floats work". They want you to buy the zine, which is what they are advertising.
> Julia Evans has written a bunch of such zines in the past as well and they are quite informative
Good for them. Why do you think this makes it not an ad?